1. Field of the Invention
The present invention relates to the reading out of sensors and in particular to the digital processing of senor output signals comprising a high-frequency carrier signal which is modulated by a measured value. In the following description of the present invention reference is made to capacitive sensors for discussing the inventive concept, like for example to micro-mechanical rotational rate sensors which use the Coriolis power for determining a rotational rate to be detected.
2. Description of the Related Art
Capacitive sensors, like for example micro-mechanical rotational rate sensors, have a variety of application opportunities. Thus, micro-mechanical rotational rate sensors are for example used in robots and mounting systems, in medical technology, in cameras for image stabilizing, in navigation systems, for stabilizing and remote-controlling road and air vehicles and also in airbag and protection systems. In general, such sensors have a movable mechanical structure which is excited to a periodical oscillation. This periodical oscillation generated by excitement is also referred to as primary oscillation. If the sensor is subjected to a rotation around an axis perpendicular to the primary oscillation or the primary movement, then the movement of the primary oscillation leads to a Coriolis force which is proportional to the measurand, i.e. the angular velocity. By the Coriolis force a second oscillation orthogonal to the primary oscillation is excited. This second oscillation which is orthogonal to the primary oscillation is called secondary oscillation. The secondary oscillation, which is also referred to as detection oscillation, may for example be detected by a capacitive measurement method, wherein the capacitively detected measurand serves as a measure for the rotational rate operating on the rotational rate sensor.
In micro-technical sensors, thereby the electronic signal evaluation is of a great importance, as the performance of the overall sensor system is to a large extend determined by the used read out and evaluation electronics. Due to the small dimensions of the micro-mechanical structure of current rotational rate sensors, like for example the rotational rate sensor DAVED®, which was developed by the Institute for Micro and Information Technology of the Hahn-Schickard-Gesellschaft e.V., very small capacities and capacity changes, respectively, down to a range of about 10−18 F must be detected, so that only very small voltages are received as sensor output signals which may, however, not be evaluated directly.
In micro-mechanical rotational rate sensors this sensor output signal is mainly limited by the noise of the electronic components of the evaluation electronic, as the actual information which is contained within the sensor output signal of the rotational rate sensor may generally not be differentiated from noise below a certain level and may then not be detected anymore.
FIG. 6 shows the block diagram of an exemplary prior capacitive sensor arrangement 300 in the form of a capacitive rotational rate sensor with a connected electronic signal evaluation arrangement 320 to detect and evaluate the capacitively detected measurand, i.e. the rotational rate acting on the sensor arrangement 300.
The capacitive sensor element 300 schematically illustrated in FIG. 6 comprises three input electrode pairs, wherein at the driver input electrode pair 302, 302′ driver input signals S1, S′1 with the frequency ω+drive, ω−drive are entered, at the electrode pair 304, 304′ primary carrier signals S2, S′2 with the frequency ω+CP, ω−CP are entered, and at the electrode pair 306, 306′ secondary carrier signals S3, S′3 with the frequency ω+CS, ω−CS are entered. The indices (+/−) thereby indicate a phase-shifting of the respective signals of 180°, so that the signal S1 is phase-shifted by 180° to S′1, the signal S2 is phase-shifted by 180° to S′2 and the signal S3 by 180° to S′3. Due to internal modulation processes within the sensor element, the actual sensor output signal at the output 308 of the sensor element is an analogue signal, which comprises the information about the detected measurand, e.g. about the present rotational rate, wherein the analogue signal comprises a carrier signal with a carrier frequency ωC, which is modulated by the measurand.
By the use of high-frequency carrier signals, the signal/noise ratio of the sensor output signal may be improved significantly, wherein in the realization of the signal evaluation electronic with analogue components, the recovery of the useful signal from the amplitude-modulated sensor output signal is performed by a double demodulation in the signal evaluation electronic 320.
The amplitude-modulated sensor output signal is thereby supplied to an (analog) operation amplifier 322 for an amplification. The amplified sensor output signal is then supplied to a high-pass filter 324 to filter out a constant component, like e.g. a DC-offset of the operation amplifier and low-frequency proportions, like e.g. ωdrive, 2*ωdrive, of the analogue sensor output signal. The waveform of the amplified analogue sensor output signal is illustrated as the waveform S4 in FIG. 6. The output signal of the high-pass filter 324 is supplied to a first demodulator (multiplier I) 326, which realizes a first demodulation (multiplication) of the signal S4 using the high-frequency carrier signal S3. This multiplication is realized by a four-quadrant differential amplifier which uses both half-waves of the input signal for multiplication. As a result a sinusoidal alternating signal S5 is obtained wherein its amplitude is directly proportional to the detected measurand, i.e. to the rotational rate.
Subsequently, the signal S5 is supplied to a second demodulator (multiplier II) 330 which converts the sinusoidal signal S5 into a direct current signal or a direct current voltage S6, respectively, which is directly proportional to the amplitude of the alternating current signal and therefore proportional to the measurand. This multiplication is performed with a low-frequency DC-voltage which is phase-shifted to the driver voltage S1.
To explain the above-described known method for reading out and evaluating an analogue sensor output signal in more detail and to be able to compare the same more easily to the inventive read out and evaluation concept later, the principle of the read out and evaluation method according to the prior art is illustrated in a summarized way again in FIG. 5.
The carrier signal ωC (e.g. 500 kHz) is fed into the capacitive sensor 300 in the middle by a signal source 310. The signal source 310 is an oscillator with a carrier and reference signal generation. The output signal of the sensor 300 is read out differentially and amplified within the operation amplifier 322. The amplified output signal is then supplied to the multiplier 326 which demodulates the amplified analogue sensor output signal by multiplying the same with the reference signal (500 kHz) from the signal source 310. The waveform S5 (see FIG. 6) is then applied to the output of the multiplier 326.
A major problem referring to this arrangement is, that the first demodulation of the sensor signal has to be performed with the high-frequency carrier signals (e.g. 500 kHz). In an oversampling of the carrier signals, a digital signal processor therefore had to work with a clock frequency which is higher than double the carrier frequency, which may not be realized reasonably with current digital signal processors due to the very extensive calculation operations that would occur.
A further problem regarding the above-described conventional sensor arrangement is, that in addition to the inherent noise of the first (analog) operation amplifier 322 further noise proportions and among others temperature drift is introduced into the useful signal by the electronic evaluation components, whereby the resolution and therefore the sensitivity and measurement accuracy of the sensor arrangement is affected significantly. This therefore leads to an operation performance in processing an output signal of a capacitive sensor arrangement which is not optimum.